Acoustic ink printing is a promising direct marking technology because it does not require the nozzles or the small ejection orifices which have caused many of the reliability and pixel placement accuracy problems that conventional drop on demand and continuous stream ink jet printers have suffered.
It has been found that acoustic ink printers embodying printheads comprising acoustically illuminated spherical focusing lenses can print precisely positioned pixels (i.e., picture elements) at resolutions which are sufficient for high quality printing of relatively complex images. See, for example, the copending and commonly assigned U.S. patent applications of Elrod et al, which were filed Dec. 19, 1986 under Ser. Nos. 944,490, 944,698, and 944,701 on "Microlenses for Acoustic Printing", "Acoustic Lens Arrays for Ink Printing" and "Sparse Arrays for Acoustic Printing", respectively. It also has been discovered that the size of the individual pixels printed by such a printer can be varied over a significant range during operation, thereby accommodating, for example, the printing of variably shaded images. See, another copending and commonly assigned U.S. patent application of Elrod et al, which was filed Dec. 19, 1986 under Ser. No. 944,286 on "Variable Spot Size Acoustic Printing".
Although acoustic lens-type droplet ejectors currently are favored for acoustic ink printing, alternatives are available; including (1) piezoelectric shell transducers, such as described in Lovelady et al U.S. Pat. No. 4,308,547, which issued Dec. 29, 1981 on a "Liquid Drop Emitter", and (2) interdigitated transducers(IDT's), such as described in a copending and commonly assigned Quate et al U.S. patent application, which was filed Jan. 5, 1987 under Ser. No. 946,682 on "Nozzleless Liquid Droplet Ejectors" as a continuation of application Ser. No. 776,291 filed Sept. 16, 1985 (now abandoned). Furthermore, the known droplet ejector technology can be adapted to a variety of printhead configurations; including (1) single ejector embodiments for raster scan printing, (2) matrix configured ejector arrays for matrix printing, and (3) several different types of pagewidth ejector arrays, ranging from (i) single row, sparse arrays for hybrid forms of parallel/serial printing to (ii) multiple row staggered arrays with individual ejectors for each of the pixel positions or addresses within a pagewidth image field (i.e., single ejector/pixel/line) for ordinary line printing.
Each of the droplets ejectors of an acoustic ink printer typically launches a converging acoustic beam into a pool of liquid ink, with the angular convergence of this beam being selected so that it comes to focus at or near the free surface (i.e., the liquid/air interface) of the ink. Printing is performed by modulating the radiation pressure which each beam exerts against the free surface of the ink. More particularly, the modulation enables the radiation pressure of each beam to make brief, controlled excursions to a sufficiently high pressure level to overcome the restraining force of surface tension, whereby individual droplets of ink are ejected from the free surface of the pool of ink on command, with sufficient velocity to deposit them on a nearby recording medium.
Unfortunately, the performance of these acoustic ink printers tends to fall off sharply as a function of any significant variance of the free surface of the ink from the output focal plane of the droplet ejector or ejectors. Known droplet ejectors characteristically have a shallow depth of focus, so the depletion of the ink supply that occurs while images are being printed can reduce the level of its free surface sufficiently to noticeably degrade the printer performance, unless suitable provision is made to compensate for the depletion of the ink. Various liquid level control systems may be employed for that purpose, but an economical and reliable solution to this control problem is needed.